In femoral joint members of conventional hip prostheses the joint head is connected via a joint neck to a shank adapted to be introduced in the medullary canal of the femur. The elongated shank which is wedge-like in the lateral-medial plane is either secured in the femur by a jamming effect and/or by means of bone cement. In both cases the medullary canal has to be prepared by a drilling operation in order to allow the insertion of the shank. A flange-like collar between the prosthesis neck and the shank engages a corresponding surface of the proximal femur and serves for the support of the prosthesis. The force transfer from the joint head of the prosthesis to the femur substantially takes place through the prosthesis shank.
Follow-up studies brought about that after a period of ten years after the implantation the loosening rate reaches 35% of the patients operated. Patients operated on before the age of 30 years sustain a loosening of up to 57% within the five years following the implantation. Loosened prostheses cause pain and require hazardous reoperations.
The main reason for the loosening of the prostheses is seen in the different modulus of elasticity of the bone material and the prosthesis shank, respectively. Due to oscillating loads so-called micro movements occur in the prosthesis shaft leading to a micro-fatigue fracture at the interface of both materials.
The stress pattern of the normal femur changes by the implantation of a prosthesis Portions of the femur normally allowing extreme loads are not loaded in the same amount after a prosthesis has been implanted. Consequently a remodeling of the bone structure happens which also contributes to a loosening of the prosthesis. Additionally, the drilling operation for the insertion of the shank considerably weakens the portions above the proximal medial femural cortex which normally has a high load capacity.
Attempts have been made to obviate the mentioned problems by allowing an ingrowth of bone material in corresponding parts of the prosthesis. For this purpose the surface of the prosthesis is provided with irregularities, openings or the like or by developing a bone cement such that it allows an ingrowth of the bone material. These attempts are not completely satisfactory.